XFP is a standard that specifies the mechanical and electrical interface specifications between an optical transceiver module and a physical layer (PHY) device. It is desirable to be XFP compliant to ensure consumers can use transceivers from various manufacturers.
Various XFP compliant transceiver modules, connectors and test PCB boards are commercially available. However, when these parts are integrated into a system, the overall resulting device is non-compliant with XFP. No solution for providing a line card with an XFP compliant 10 Gbps optical port is currently known.
A printed circuit board (PCB) utilizes traces to electrically connect different electronic components. The traces are conductive pathways of copper used for relaying electrical signals. The traces are laminated onto a non-conductive substrate to form the PCB. The non-conductive substrate could include glass or other insulating materials.
Typically, the PCB is formed by using a blank PCB with a layer of copper covering the entire substrate and removing all the undesired copper to form the copper traces. However, copper traces can also be added to the substrate to form the PCB in certain cases.
Vias are holes through a PCB. The walls of the vias are copper plated to electrically connect the conducting layers of the PCB. Electronic components may be attached to pads on the outer surfaces of the PCB. Component leads can be fixed to the board via soldering.
Electromagnetic interference (EMI) is a disturbance caused in an electrical circuit by electromagnetic radiation. The disturbance can interrupt or degrade the circuit performance. Bypass capacitors, resistors, filtering, and shielding with EMI gaskets may be used to minimize these disturbances.
A line card is simply an electronic circuit on a PCB that interfaces with the telecommunications lines of subscribers to the telecommunications access network. As the name suggests, a transceiver is a device having both transmitter and receiver circuitry. Differential pairs are conductors found on PCBs for carrying differential signals.
At 10 gigabits per second (Gbps), even very tiny non-idealities in a PCB can have significant impact on the resulting signal. XFP specifications require a PCB footprint which constrains break-out options. The presence of the ground shield prevents component-side routing breakout. Routing near the ground requires minimizing impedance discontinuities and ground coupling.
One solution for component-side break out includes removing the EMI gasket to avoid making contact with the differential pair. The solder mask is prone to wear over time, however, and the close proximity of the XFP ground shield is also a concern.
A solder-side break-out solution is to drop vias in the XFP connector pin field and route on the solder side of the board. However, the undesirable and parasitic properties of the vias make meeting the electrical specifications (i.e. return loss) very difficult to achieve.
Hence, there is a need in the art for a convenient to use, inexpensive and efficient PCB interconnect for the XFI interface between a 10 Gbps PHY and an XFP transceiver meeting all of the requirements of the XFP standard.